1. Field of the Invention
The present invention relates generally to the field of testing of integrated circuits, and more specifically to a sequential scan technique which leads to reduction of one or more of test time, cost and power consumption.
2. Related Art
Sequential scan techniques are often used to test integrated circuits. According to a typical sequential scan technique, integrated circuits are designed to operate in functional mode or test mode. In functional mode, elements in the integrated circuit are connected according to a desired design and to provide a desired utility for which the integrated circuit is primarily designed.
In test mode, the integrated circuit is designed to connect various memory elements (contained in the integrated circuit) such as flip-flops in a sequence referred to as a “scan chain” (i.e., the output of one element is connected as an input to the next element). The first element in the scan chain is generally designed to receive the input bits and the last element of the scan chain is designed to scan out the results of evaluation, as described below.
In a typical scan test scenario, a number of bits in a particular pattern of zeros and ones (scan vector) are sequentially (one bit at every clock cycle) loaded (scanned in) into scan chain through the first element. The number of bits contained in the scan vector generally equals the number of memory elements in a corresponding scan chain.
Once a scan chain is loaded with a scan vector, the elements (generally the combinatorial logic) in the integrated circuit are evaluated based on the scanned in bits. The flip-flops are designed to latch the results of the evaluation, and the bits latched in the scan chain are sequentially scanned out (one bit at every clock cycle) through the last element in the scan chain. The received scan out is compared with an expected scan out corresponding to the scan vector to determine the various faults within the integrated circuit.
Multiple scan chains are present, particularly in very large scale integrated circuits. Each scan chain covers a corresponding portion of the integrated circuit, and thus may need to be tested with a corresponding set of scan vectors. Accordingly, when a scan chain is loaded and tested with a scan vector, a desired controllability (of the internal logic of the portion being tested) and observability (visibility of the corresponding accuracy of operation) of the corresponding portion is obtained.
In general, such tests need to be performed meeting at least some of several requirements. Some of the such requirements include minimizing the testing time, reducing power dissipation during test time, etc. Reducing the testing time generally leads to corresponding lower costs since the usage of testing equipment (and labor type overhead) is reduced. Reduction of power dissipation is of concern since substantially more power dissipation can occur in test mode compared to functional mode, and integrated circuits may be designed with a power dissipation specification corresponding to only the functional mode.
What is therefore needed is a scan test model for testing integrated circuit with reduced power, time and/or cost.
In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.